The heart is a mechanical pump that is stimulated by electrical impulses. The mechanical action of the heart results in blood flow through a person's body. During a normal heartbeat, the right atrium (RA) of the heart fills with blood from veins within the body. The RA then contracts and blood is moved into the heart's right ventricle (RV). When the RV contracts, blood held within the RV is then pumped into the lungs. Blood returning from the lungs moves into the heart's left atrium (LA) and, after LA contraction, is pumped into the heart's left ventricle (LV). Finally, with the contraction of the left ventricle, blood from the LV is pumped throughout the body. Four heart valves keep the blood flowing in the proper directions during this process.
The electrical signal that drives the heart's mechanical contraction starts in the sino-atrial node (SA node). The SA node is a collection of specialized heart cells in the right atrium that automatically depolarize (change their potential). The depolarization wavefront that emanates from the SA node passes across all the cells of both atria and results in the heart's atrial contractions. When the advancing wavefront reaches the atrial-ventricular (AV node), it is delayed so that the contracting atria have time to fill the ventricles. The depolarizing wavefront then passes across the ventricles, causing them to contract and to pump blood to the lungs and body. This electrical activity occurs approximately 72 times a minute in a normal individual and is called normal sinus rhythm.
Abnormal electrical conditions can occur that can cause the heart to beat irregularly; these irregular beats are known as cardiac arrhythmias. Cardiac arrhythmias fall into two broad categories: slow heart beats or bradyarrhythmia and fast heart beats or tachyarrhythmia. These cardiac arrhythmias are clinically referred to as bradycardia and tachycardia, respectively.
Bradycardia often results from abnormal performance of the AV node. During a bradycardial event, stimuli generated by the heart's own natural pacemaker, the SA node, are improperly conducted to the rest of the heart's conduction system. As a result, other stimuli are generated, although their intrinsic rate is below the SA node's intrinsic rate. Clinical symptoms associated with bradycardia include lack of energy and dizziness, among others. These clinical symptoms arise as a result of the heart beating more slowly than usual.
Bradycardia has been treated for years with implantable pacemakers. Their primary function is to monitor the heart's intrinsic rhythm and to generate a stimulus strong enough to initiate a cardiac contraction in the absence of the heart's own intrinsic beat. Typically, these pacemakers operate in a demand mode in which the stimulus is applied only if the intrinsic rhythm is below a predetermined threshold.
Tachycardia often progresses to cardiac fibrillation, a condition in which synchronization of cell depolarizations is lost, and instead, there are chaotic, almost random electrical stimulations of the heart. Tachycardia often results from ischemic heart disease in which local myocardium performance is compromised and coordinated contraction of heart tissue is lost which leads to a loss of blood flow to the rest of the body. If fibrillation is left untreated, brain death can occur within several minutes, followed by complete death several minutes later.
Application of an electrical stimulus to a critical mass of cardiac tissue can be effective to cause the heart to recover from its chaotic condition and resume normal coordinated propagation of electrical stimulation wavefronts that result in the resumption of normal blood flow. Thus, the application of an electrical stimulus can revert a patient's heart to a sinus cardiac rhythm and the chambers of the heart once again act to pump in a coordinated fashion. This process is known as defibrillation.
Cardioversion/defibrillation is a technique employed to counter arrhythmic heart conditions including some tachycardias in the atria and/or ventricles. Typically, electrodes are employed to stimulate the heart with high energy electrical impulses or shocks, of a magnitude substantially greater than the intrinsic cardiac signals. The purpose of these high energy signals is to disrupt the generation of the chaotic cardiac signals and cause the heart to revert to a sinus rhythm.
There are two kinds of conventional cardioversion/defibrillation systems: internal cardioversion/defibrillation devices, or ICDs, and external automatic defibrillators, or AEDs. An ICD generally includes a housing containing a pulse generator, electrodes and leads connecting the electrodes to the housing. Traditionally, the electrodes of the ICD are implanted transvenously in the cardiac chambers, or alternatively, are attached to the external walls of the heart. Various structures of these types are disclosed in U.S. Pat. Nos. 4,603,705; 4,693,253; 4,944,300; 5,105,810; 4,567,900; and 5,618,287, all incorporated herein by reference.
In addition, U.S. Pat. Nos. 5,342,407 and 5,603,732, incorporated herein by reference, disclose an ICD with a pulse generator implanted in the abdomen and two electrodes. In one embodiment (FIG. 22), the two electrodes 188, 190 are implanted subcutaneously and disposed in the thoracic region, outside of the ribs and on opposite sides of the heart. In another embodiment (FIG. 23), one electrode 206 is attached to the epicardial tissues and another electrode 200 is disposed inside the rib cage. In a third embodiment (FIG. 24), one electrode 208 is disposed away from the heart and the other electrode 210 is disposed inside the right ventricle. This system is very complicated and it is difficult to implant surgically.
Recently, some ICDs have been made with an electrode on the housing of the pulse generator, as illustrated in U.S. Pat. Nos. 5,133,353; 5,261,400; 5,620,477; and 5,658,325, all incorporated herein by reference.
ICDs have proven to be very effective for treating various cardiac arrhythmias and are now an established therapy for the management of life threatening cardiac rhythms, such as ventricular fibrillation. However, commercially available ICDs have several disadvantages. First, commercially available ICDs must be implanted using somewhat complex and expensive surgical procedures that are performed by specially trained physicians. Moreover, lead placement procedures require special room equipped for fluoroscopy. These rooms are limited in number and therefore, limit the number of lead placement procedures, and ultimately the number of ICDs, that may be implanted in any given day.
Second, commercially available ICDs rely on transvenous leads for the placement of at least one electrode within the cardiac chambers. It has been found that over a period of time, transvenous lead electrodes may get dislodged from the cardiac tissues. Additionally, complications such as broken leads and undesirable tissue formations deposits on the electrodes are not uncommon. These problems are especially acute when leads carry two or more electrodes. Moreover, infection is a concern when implanting leads within a patient's vasculature.
Third, removing these ICDs and replacing them, if necessary, also requires complicated surgical procedures that may be more life-threatening than the initial implantation.